If you have online banking access already, it's displayed there. I always have to login to online banking whenever I'm buying something over where they need a/c no and sort code because, as mentioned, Natwest don't put them on the card any more...

yeah best to just ask your branch - but normally the account number is not on the card for banks - it is normally only actually printed on the card with building society accounts (eg my nationwide card has it printed, but not my RBS)

Should be on any statements/paying in slips though, and I would expect it to be printed on any correspondence between the bank and yourself...but I guess not. Your branch should tell you though